Patients with severe asthma have chronic airflow limitation that is partly due to increased airway smooth muscle tone and structural changes of the airway wall. In addition, such patients appear to have lost a major protective function of their lungs, the ability of lung infaltion (through deep inspiration) to reverse airway tone and to bronchodilate. This ability is strong in halthy subjects and only minimally reduced in mild asthmatics. Using HRCT, our group has observed that lung inflation distends the airways of healthy subjects and subjects with mild asthma. However, a decrease in airways distensibility has been observed in subjects with chronic airflow limitation. There are two possible causes for reduced airways distensibility, increased airway tone and/or structural wall changes. These structural wall changes have a reversible and irreversible component. The reversible component is believed to be sensitive to steroids.